Chemical vapor deposition (CVD) is a process of forming a film on a substrate, typically, by generating vapors from liquid or solid precursors and delivering those vapors to the surface of a heated substrate where the vapors react to form a film. Systems for chemical vapor deposition are employed in applications such as semiconductor fabrication, where CVD is employed to form thin films of semiconductors, dielectrics and metal layers. Three types of systems commonly used for performing CVD include bubbler-based systems, liquid-mass-flow-control systems, and direct-liquid-injection systems.
Bubbler-based systems, or xe2x80x9cbubblers,xe2x80x9d essentially bubble a stream of gas through a heated volume of liquid precursor. As the stream of gas passes through the liquid precursor, vapors from the liquid precursor are absorbed into the gas stream. This mixture of gases is delivered to a process chamber, where the gases react upon a surface of a heated substrate. Bubblers typically heat the volume of liquid precursor at a constant temperature. Over time, the constant heat often causes the precursor to decompose rendering it useless for CVD. In an effort to minimize decomposition, the bubbler is typically maintained at a temperature lower than that at which the vapor pressure of the liquid precursor is optimal.
Liquid mass flow control systems attempt to deliver the precursor in its liquid phase to a vaporizer typically positioned near the substrate. The precursor is vaporized and is then typically entrained in a carrier gas which delivers it to the heated substrate. A liquid mass flow controller, which is a thermal mass flow controller adapted to control liquids, is used to measure and control the rate of flow of liquid precursor to the vaporizer.
Liquid mass flow controllers present a number of drawbacks. First, liquid mass flow controllers are extremely sensitive to particles and dissolved gases in the liquid precursor. Second, liquid mass flow controllers are also sensitive to variations in the temperature of the liquid precursor. Third, liquid mass flow controllers typically use a gas to assist in the vaporization of the liquid precursor, thereby increasing the probability of generating solid particles and aerosols and ensuring a high gas load in the process system. Fourth, most liquid mass flow controllers cannot operate at temperatures above 40xc2x0 C., a temperature below which some precursor liquids, such as tantalum pentaethoxide (TAETO), have high viscosity. Due to its sensitivities, the liquid flow controller is accurate and repeatable to about 1% of full-scale liquid flow. Further, when a liquid mass flow controller wetted with TAETO or one of a number of other precursors is exposed to air, the precursor will generally react to produce a solid which may destroy the liquid flow controller.
Liquid pump-based systems pump the liquid precursor to the point of vaporization, typically at a position near the heated substrate. Liquid pump-based systems are generally one of two main types. One type uses a liquid flow meter in line with a high-pressure liquid pump. The other type uses a high-precision, high-pressure metering pump. Both of these systems are extremely sensitive to particles in the liquid. The liquid-flow-meter based system is also sensitive to gas dissolved in the liquid. Both are extremely complex to implement, and neither can tolerate high temperatures (maximum 50xc2x0 C.). The system with the metering pump has difficulty vaporizing high viscosity liquids. Finally, both are generally difficult to implement in a manufacturing environment due to their extreme complexity and large size.
Existing CVD equipment design is generally optimized for high process pressures. The use of high process pressures is most likely due to the fact that, until recently, CVD precursors were either generally relatively high-vapor-pressure materials at room temperature or were, in fact, pressurized gases. Examples include tetraethyloxy silicate (TEOS), TiCl4, Silane, and tungsten hexafluoride, etc. These materials were chosen because they had high vapor pressures and could therefore be easily delivered. The process pressure was generally well within the stable vapor pressure range of each of these materials.
The present invention relates to systems and methods for chemical vapor deposition for the fabrication of materials and structures for a variety of applications. The system is well suited for use in the fabrication of devices for the semiconductor industry, but can also be used in other applications involving thin film deposition and processing.
In addition to the fabrication of dielectric layers, metalization layers, and epitaxially grown semiconductor films including silicon, germanium, II-VI and III-V materials, the system can be used for precision manufacture of optical thin films such as anti-reflective coatings or stacked dielectric structures including optical filters, diamond thin films or composite structures for multichip modules or optoelectronic devices.
In contrast to thin films of traditional CVD materials, future thin films require new materials that have low vapor pressures and that are often near their decomposition temperature when heated to achieve an appropriate vapor pressure. Some of the precursors having both intrinsically low vapor pressure and low thermal decomposition temperature are considered the best choices for deposition of films of tantalum oxide, tantalum nitride, titanium nitride, copper, and aluminum.
An apparatus of this invention includes a vaporizer within a vaporization chamber and a dispenser positioned for dispensing a precursor to the vaporizer. A delivery conduit joins the vaporization chamber with a process chamber, where a chemical vapor is deposited on a substrate. A flow meter is positioned to measure vapor flow through the delivery conduit, and a flow controller is positioned to control vapor flow through the delivery conduit. Both the flow meter and flow controller are communicatively coupled with a processor programmed to control the flow controller to govern vapor flow through the delivery conduit in response to the measured vapor flow.
In a preferred embodiment, the flow meter includes a tube with a pair of open ends, which acts as a laminar flow element. The flow meter further includes a pair of capacitance manometers aligned with the open ends of the tube to measure the pressure drop across the laminar flow element. In a further preferred embodiment, the flow controller is a proportional control valve in communication with the flow meter.
A still further preferred embodiment of the apparatus includes a reservoir for supplying precursor to the dispenser. The dispenser is controlled by the processor and the vaporizer which receives precursor from the dispenser includes a heated surface for vaporizing the precursor. Preferably, a pressure sensor communicatively coupled with the processor is positioned in the vaporization chamber. Accordingly, the processor can, in some embodiments, control the rate at which vapor is generated by the vaporizer, by, for example, controlling the rate at which the dispenser dispenses precursor from the reservoir to the vaporizer.
In another embodiment of the apparatus, an outlet of the delivery conduit is positioned in the process chamber, and a showerhead divides the process chamber into an upstream section and a downstream section, wherein the outlet is in the upstream section and a substrate chuck is in the downstream section. An upstream pressure sensor is positioned to measure vapor pressure in the upstream section, and a downstream pressure sensor is positioned to measure vapor pressure in the downstream section. Both the upstream and downstream pressure sensors are communicatively coupled with a processor. In a further preferred embodiment, the showerhead is xe2x80x9cactive,xe2x80x9d enabling control over the vapor flow rate through the showerhead.
Other features found in preferred embodiments of the apparatus include a heater in thermal contact with the delivery conduit, a DC or AC source connected to the substrate chuck, and an elevator for raising and lowering the substrate chuck. Another embodiment of this invention is a cluster tool for semiconductor processing including a CVD apparatus, described above, connected to a central wafer handler.
In a method of this invention, a precursor is vaporized in a vaporization chamber, gas flow between the vaporization chamber and a process chamber is measured, and the rate of gas flow between the vaporization chamber is controlled in response to the measured gas flow. In another embodiment of a method of this invention, the vapor pressure of a precursor is measured, and the rate at which the precursor is vaporized is controlled in response to the measured vapor pressure, preferably by controlling the rate at which precursor is dispensed from a reservoir onto a vaporizer. Preferably, deposition occurs via a surface-driven reaction. Nevertheless, embodiments of the invention also cover methods where deposition occurs via non-surface driven reactions.
The systems and methods of this invention provide numerous benefits. First, they allow the precursor to be delivered to the substrate in a much purer and higher-pressure or high-flux form than is achievable with the use of systems that use a carrier gas. As a result, the likelihood of gas-phase reactions and consequent formation of particles can be greatly reduced. Because of the higher concentration, which leads to a higher deposition rate, this invention does not necessitate the introduction of plasma into the process chamber. Consequently, the apparatus is simplified, and plasma-induced polymerization of precursor is reduced or eliminated. Second, control over the concentration of precursor delivered to the process chamber is enhanced, thereby improving control over film thickness and uniformity. Third, the direct delivery of vapor flow into the process chamber at low temperature and low pressure and without a carrier gas increases the efficiency of use of costly precursors in many applications by a factor of up to 10 or more over standard systems utilizing a carrier gas, which infer precursor vapor flow rates either from a theoretical pickup rate, which is carrier-gas and temperature dependent, or from a thermal mass-flow controller or liquid delivery system. Likewise, emissions of unreacted process gases from the process chamber can be maintained at very low levels because the absence of a carrier gas and generally lower flow rates and better residence times leads to a higher utilization efficiency of the precursor. Fourth, decomposition of the precursor is limited due to its short contact time with the heated vaporizer. While small amounts of precursor are delivered to the vaporizer, as needed, the useful life of the bulk of the precursor is preserved by maintaining it at a lower temperature in the reservoir. Fifth, the highly conformal nature of deposits that can be formed by methods of this invention are useful in forming integrated circuits with line-widths of 0.25 microns (250 nm) or less.
Other advantages of this invention include the low sensitivity of the system to impurities such as dissolved gases and particles in the precursor, the relative ease of alternating between multiple precursors in a single system as a result of the ability to coordinate the use of each with a common precursor delivery system, the ease of accessing and maintaining all subsystems, the low power requirements of the system, the use of only low voltages in the operating elements of the system and the small overall size of the system.